The present invention relates generally to an apparatus for generating a pulse-like constant current having a variable pulse width which can be profitably used for evaluating thermal behaviors or characteristics of laser diodes. More particularly, the invention is concerned with an apparatus which includes a first high-speed amplifier and a second low-speed amplifier connected in parallel with each other for generating constant current pulses with variable pulse width or duration by controlling the on/off durations of the first and second amplifiers.
As one of the techniques in which the pulse-like constant current is made use of, there may be mentioned a test for evaluating thermal characteristics or behaviors of the laser diodes.
The laser diode is an element which is susceptible to the influence of heat such that oscillation wavelength thereof is shifted to a region of long wavelengths and/or the oscillation threshold current is increased as the temperature rises, involving decrease in the output laser power as well as in the use life of the laser diode. The temperature rise of the laser diode is generally in proportion to the magnitude of an excitation current applied to the laser diode for light emission and a period during which the excitation current is supplied. Accordingly, in order to ensure a stabilized operation of the laser diode, such a structure is required which can suppress the temperature rise brought about by the current injected for exciting the light emission to a minimum possible level and which can allow the heat generated by the laser diode in the light emitting state to be released externally.
As the method of evaluating the thermal characteristics or behaviors of the laser diode in the operating state, there is known a method of measuring and comparing changes or variations in the light emission characteristics by changing the period for driving or exciting the laser diode by applying selectively a pulse-like current of a short pulse width (duration) and a pulse-like current of a long pulse width to the laser diode.
The present invention is directed to an apparatus for generating a pulse-like constant current to be utilized in evaluation of the thermal characteristics of the laser diodes.
As a prior art pulse-like constant current generating technique, there has been proposed an approach in which a feedback circuit is employed, as is disclosed, for example, in Japanese Patent Application Laid-Open No. 63834/1987 (JP-A-62-63834) (in particular, reference may be made to FIGS. 3 and 4). More specifically, there are provided in the output stage of the pulse-like constant current generator mentioned above a pluralitY of high-speed switching transistors which are connected in parallel. However, the transistors of this sort are generally limited in respect to the permissible collector loss and can operate only with a narrow output pulse width in a region where the duty ratio of the output current is smaller than several percentages, thus giving rise to a problem. Such being the circumstances, when the output current is to be generated with an increased pulse width, a large number of transistors constituting major parts of the output stage of the current generator are employed in a parallel connection. The current generator of such circuit configuration nevertheless suffers from disadvantages that the stray loss is increased, involving instability in operation, not to speak of high expensiveness.
In the followings, the prior art pulse-like current generator will be described in concrete by referring to FIGS. 5, 6 and 7 of the accompanying drawings.
In FIG. 5, a reference numeral 21 denotes an input voltage terminal, 22 denotes a terminal for a timing signal, 23 denotes an analogue switch, 24 to 27 denote amplifiers, respectively, and a numeral 28 denotes a laser diode.
The amplifier 25 is constituted by three transistors connected in parallel. On the other hand, the amplifiers 24, 26 and 27 cooperate to constitute a feedback circuit, whereby a constant current circuit is realized.
Under the timing commanded by the timing signal applied to the terminal 22, the analogue switch 23 is turned on and off, whereby a rectangular pulse current is supplied to the laser diode 28. The output characteristics of the constant current circuit shown in FIG. 5 are illustrated in FIGS. 6 and 7. As can be seen in FIG. 6, the current I.sub.F drops or decreases significantly when the pulse width or duration exceeds 10,000 .mu.S at the duty ratio of 0.1%, which means that increasing the pulse width beyond a predetermined value renders it no longer possible to derive the constant current output, presenting a problem. On the other hand, the current I.sub.F decreases as a function of the increase in the duty ratio, as is illustrated in FIG. 7. In other words, there exists another problem that a large constant current can not be derived as the output current when the duty ratio is increased.